Metal bracket with screw for its clamping

ABSTRACT

Improvements to the construction of metal brackets with screw for the clamping or tightening thereof, the vertices of the extremities of the two arms of the front cross-bar at the body-tunnel entrance are enlarged, and a space is provided for the housing thereof into the screw body, with concentric bearing flange and double clamping tailpiece of the two ends of the metal strip coiled from the tunnel base being substituted by a transverse monotailpiece which is rabated to the opposite side, with a threaded tongue for clasping purposes, and one of the extremities of the strip having two juxtaposed drawn pieces, one in the upper surface and the other one in the lower surface of crimping with the edge of the transverse monotailpiece, the upper one impeding the excessive elevation of the proper extremity.

The subject of the present invention relates to improvements made in theconstruction of metal clips or clamps with tightening screw for securingthem.

This type of clip is known at present, the clips being essentiallyconstituted by a metal strip having permanently joined to one of itsends a tunnel-like body to house the screw.

The metal strip constituting the clip has near the opposite end to thatdescribed a multiplicity of transverse, inclined slots, parallel to oneanother, which do not reach the edges of the longitudinal sides of thestrip and act as the teeth of a rack.

Such clips are assembled by coiling the metal strip onto itself, forminga loop, and the free end of the strip is inserted into the spaceexisting between the open base of the body of the tunnel and the surfaceof the end to which the tunnel carrying the tightening screw ispermanently joined.

By means of this insertion, the helical threads of the screw, projectingthrough the open base of the tunnel, mesh rotationally in the inclinedtransverse slots, parallel to one another, of the inserted free end ofthe strip, and on turning the screw in its tightening movement they pushthe intermediate walls between two slots and thus the end of the stripis pulled or pushed, depending on the direction in which the screw isturned, and by this means the constriction or loosening of the loop iseffected.

The separation between juxtaposed slots of the free end of the metalstrip coincides with the pitch of the screw threads of the screw.

Near the lower edges of the major sides of the sheet from which thetunnel carrying the screw is formed there are two longitudinal slotsparallel to the said edges for the purpose of narrowing at the saidpoint the amplitude of the separation existing between the descendingwalls of the tunnel, bringing them closer to the body of the screw, andalso in order to provide greater resistance to negative warping thereof.

The head of the screw inserted inside the tunnel body is arranged on theoutside of the inner chamber which the latter forms for housing the bodyof the tunnel screw, the said head bearing against the forward edge ofthe fixing transverse member placed on the inlet opening of the saidchamber.

The fixing transverse member is constituted by two limbs sharply bent ina direction towards each other and their free faces are opposed,touching each other.

These bent limbs start perpendicularly from the minor lateral ends ofthe stamped metal strip which forms the tunnel body for housing thetightening screw, and they are then sharply bent.

In order to achieve the above-mentioned advance of the free end of themetal strap of the clip, it is necessary to apply the couple of force tothe head of the screw, located outside the housing chamber, in aclockwise direction, so that the rear surface of the said head pressesfirmly against the edge of the fixing transverse member of the screwhousing, and at the same time entrains, with its helical screw threads,the end of the metal strip.

With the strong transverse pressure of the rear part of the outer headof the screw which has its body housed inside the chamber during thetightening phase, it often occurs that the ends of the transverse limbs,sharply bent towards each other, which constitute the fixing transversemember, are forcibly displaced laterally, which is detrimental to theproper functioning of the device.

It is a consequence of its narrowness that it offers insufficientresistance to the strong perpendicular thrust of the rear part of thehead of the screw.

The negative lateral displacement of the ends of the bent limbs preventsthe correct rotational coaxial sliding of the screw as it moves forwardsor backwards, resulting in lateral deviation of the screw from thecoaxial line of its housing, and as a consequence of this all the forceof the load applied against the fixing transverse member is convertedinto an asymmetrical force, and therefore there arises the negativedeviation from the parallel of the body of the screw in relation to theinner wall of the housing tunnel.

The above-mentioned deviation of the body of the screw deforms, underpressure, the actual casing of its housing and, when the action oftightening the screw is continued, this deformation incorrectly allowsthe entry of the head of the screw itself, and its larger dimensionsthen press against the inner walls of the tunnel, deforming them evenmore and causing the body of the tunnel to break and become detachedfrom the point at which it is connected to the appropriate end of thestrip.

Therefore, since there is the possibility that during the operation ofthe clip the screw, in its tightening phase, may adopt an inclinedposition relative to the axis of the tunnel, it is normal and currentpractice in the industry manufacturing clips of this type to provide twomore rectangular slots, arranged in the transverse direction of thetunnel, placed near the edge of the front face of the tunnel forming thechamber for housing the screw.

Such additional slots each start on the side near the bending point ofthe limbs which, once bent towards each other, act as a bearing crossmember for the outer head of the screw, the body of which is inserted.

The depth of the aforesaid additional slots is greater at their point oforigin, and they decrease in depth towards the centre of the sheet,which coincides with the natural arc forming the tunnel, and theydisappear at the vertex of the latter.

These additional slots provided transversely in the body of the walls ofthe tunnel of this type of clip serve the purpose of partially andtransversely constricting the inner space behind the bent limbs,existing between the lower edges of the two sides of the inlet of thetunnel in order to bring the constricted walls closer to the shank ofthe body of the inserted screw, thus providing the latter with twoopposed points of contact and coaxial guiding.

Therefore, as regards the bent limbs, their narrowness is conditioned bythe separation existing between the surface of the rear base of the headof the screw and the termination, near the said point, of the end of thecontinuous, screw-threaded helix of the screw itself.

A consequence of this is another additional problem in that, since thebent limbs are narrow, their free ends do not have sufficient strengthto resist the very strong thrust of the head of the screw which pressesperpendicularly against their edges.

In order to avoid this problem, in the industry it has been proposed toreinforce the aforesaid bent limbs, providing them with a greaterthickness.

This solution is not possible, since it should not be forgotten that thelimbs bent in a transverse direction form part of the same laminar metalplate which is then shaped into a tunnel and therefore they can haveonly the same thickness as the remainder of the stamped sheet.

Owing to this, in the industry the transverse bent limbs continue to beproduced, without any increase in thickness and with the upper surfaceof the bent limbs having a slight inclination from the forward edge totheir opposed rear edge, having at a specific point, coinciding with thepitch between turns, a perpendicular prong arising from the rear edge ofeach limb so that it coincides in position with the pitch of the lastturn near the rear part of the inserted screw and these serve asrotational and guiding anchorage projections for the latter.

Since the perpendicular prongs are weak and small, nor are theycompletely effective for their function of rotational engagement betweenthe pitch of two turns of the screw, and owing to the strong pressureswhich develop in the said area on applying the couple of force to thehead of the screw, they break and disappear.

Moreover, the loop of the metal strip which clips of this type form,sometimes, although too frequently, continues to come off accidentallywhen the loop undoes itself, through the escape of the end insertedbetween the screw and the body coiled onto itself, is a consequence ofthe high tensile force of the screw in its actual action of constrainingthe strip forming a loop.

On the other hand, in the clip industry there have been proposeddifferent forms of fixing of the tunnel to the end of the metal strip,but frequently, owing to the said strong tensions, the joint obtained isdestroyed, the clip being rendered useless.

Registrations of this known type which may be cited, among many othernational and foreign registrations, are: Spanish Utility Model No. 281033 of the company Mikalor S.A. (corresponding to U.S. Pat. No.4,706,346 and to European Patent No. 171 481 of the said Mikalor S.A.);U.S. Pat. No. 4,473,928 of Francis W. Johnson; British Patent 708 444 ofDoye; U.S. Pat. No. 2,944,314 of Brack; U.S. Pat. No. 4,528,730 ofGeorge E. Spaulding; Dutch Patent No. 80 296 of Schendelen, and manyothers, such constructional details being different from one another andthat of the related registrations of the Applicant.

Therefore, in spite of the variations in detail provided in the relatedregistrations, in all of the clips with the head piece for housing thetightening screw, the production of the clip belonging to the said typein the public domain involves basically the same problems, more or lessacute, arising from the fact of being the said common type of clip.

With the subject of the improvements of the invention, the resultachieved is that the ends of the transverse limbs, sharply bent and withtheir faces opposite one another and contacting one another, which formthe fixing transverse member, can be constructed with a greater width,gaining in resistance to the perpendicular thrust of the rear part ofthe head of the screw during the tightening action of the latter.

This increase in width also makes it possible to increase the inclinedsurface existing between one edge and the other of the sides of the twobent limbs facing one another, and thereby obtain an improved anchorageof the ends of the two bent limbs which permit the rotational movementupon itself of the screw.

It should be borne in mind that the inclination in a transversedirection of the bent limbs is of extreme importance in this known typeof clip.

On the one hand it facilitates the introduction of the rear edge of thefixing transverse member formed by the two bent limbs into the pitchbetween turns of the screwed-in screw, and on the other hand provides aninclined surface for thrust against the end of one of the two superposedat the point of junction of the tunnel body with the end of the coiledstrip forming a loop.

It is clear that at present, because the bent limbs are narrow and haveinsufficient rotational anchorage, it is necessary to arrange on therear edge weak perpendicular prongs at the above-mentioned point ofengagement of the screw, nevertheless permitting it to turn on itself.

However, it should never be forgotten that on the edge of the fixingtransverse member at which the tightening of the rear part of the screwis effected, a strong thrust force is exerted, so that with thearrangement of such weak engagement prongs of the rear edge of the twobent limbs a weak resistance is provided compared with that strongthrust force which easily destroys the aforesaid prongs, thusre-initiating the negative lateral deviation of the ends of the sharplybent limbs.

It is clear that with the enlargement imparted to the ends of the bentlimbs, one of the improvements of this patent is that of eliminating theweak engagement prongs at the edges of the bent limbs, since it is theactual faces of said lateral edges which, owing to the greater width ofthe limbs and the improved seating for the latter provided on the bodyof the screw, can reach the annular vertex of the start of the supportcollar arranged on the neck of the body of the screw and obtain improvedsupport.

On the other hand, the annular permanently connected collar, since it isnot part of the helical screw thread, can be arranged during manufactureof the screw at a convenient height on the latter, the dimensions of thesmooth annular seating space for the lateral edge of each enlarged limbon the body of the screw thus being able to be determined as required.

In order to obtain this larger, smooth annular space mentioned, theupper final end of the helical screw thread of the body of the screw isshortened.

On the other hand, because the bearing point of the said rear edge ofeach limb is a circular collar and not a helical turn, the bearing forceagainst the collar is uniform and not unequal as occurs at present inthat the said edge bears against a helically inclined surface, that ofthe helical screw thread of the screw.

Likewise, in the improvements which are the subject of the invention,means are arranged to avoid the collapse of the walls of the bent limbswhich have a side bearing on the annular vertex, under the strongperpendicular pressure on the opposed side of the head of the screw.

Such means contribute even more to avoiding the deformation of thesurface of the sharply bent limbs which form the fixing transversemember; on the neck of the screw they consist in arranging anotherannular collar, with double slope, constituting a narrow-edged circularrim.

This annular collar is placed between the annular collar arranged at theend of the screw thread of the body of the screw and the collarpermanently joined to the rear part of the head of the screw itself.

The narrow, annular projecting edges of this additional interposedcollar, when gradually introduced, by screwing of the body of the screw,into the housing chamber of the tunnel, exerts simultaneous rotationalpressure against the inner side wall of the chamber of the tunnel andagainst the corresponding bent limb, which simultaneous double pressureprevents the deformation of the walls against which it acts.

All this manifests itself in a greater resistance to deformation whenthe strong perpendicular thrust of the rear part of the head of thescrew is produced, so that according to the improvements of theinvention the bent limbs are practically undeformable and it contributesto avoiding the problems of the deformation of the chamber housing thescrew.

Also in this type of clip there is likewise the problem that the strip,when it forms a loop and has its free end backing against the lower partof the length of strip arranged beneath the tunnel as a consequence ofthat same force of rotation of the screw for the constriction of theclip, the said end of the metal strip has a tendency to follow therotational movement and thus be displaced laterally projecting angularlyfrom beneath the upper superposed length of the strip.

This problem of lateral projection is important in a negative sense,when the clip is fitted in cramped spaces which prevent the freemovement of the hands of the operator when they are working and savingin the said cramped place, there being the annoying possibility ofproducing unpleasant scratches on the hands of the operator, as occursinside the "bonnets" of motor vehicles.

There is another important problem in the construction of clips of thistype, which is that of joining the tunnel to the end of the metal strip.

At present, in order to join the tunnel for housing the tightening screwto the corresponding end of the metal strip, a pair of equal appendagesare used which project perpendicularly from each lower edge of the sheetforming the tunnel body, and said perpendicular appendages are bent anddepressed respectively, their ends being bent and their faces oppositeone another, touching one another in the middle of the space existingbetween the walls of the open base of the tunnel body, simultaneouslyclamping the two superposed ends of the coiled metal strip forming aloop.

This composite clamping is weakened by the central dividing line formedby the opposed facets of the two facing appendages.

In order to remedy this weakening of the bent appendages in the knownconstructions, a perpendicular bracket is caused to project from thelower end of the strip, which simultaneously rises over the twojuxtaposed coinciding vertices of each end of both the transverseappendages, which vertices are flattened for improved bearing of the endof the bracket, which is depressed and simultaneously presses againstthe two flattened points provided by the two juxtaposed vertices of thetwo transverse appendages.

This additional clamping to avoid, the separation of the two ends of thebent transverse appendages is precarious owing to the strong separatingforce which is exerted on the free ends of both bent transverseappendages when the tightening screw is tightened.

However, in spite of the said precautions, it often occurs that thismanner of joining the tunnel body to the corresponding end of the metalstrip cannot prevent the negative lateral displacement of the ends ofthe said appendages.

Consequently the clip would prove to have to be replaced by another.

Owing to the improvements of the invention, the joining of the tunnelbody to the end of the metal strip is not effected in the form disclosedof the two equal bent appendages and the containment bracket, but thejoining of the tunnel body to the end of the clip which abuts the edgeof the lateral of the same side of a single transverse appendage. Andmoreover, at the other end of the actual metal strip, it has stopsprojecting from its surface tending to rise up when the metal strip ispulled, which stops arranged in the recommended manner of the inventionprevent the disengagement of the end of the strip.

Also in the improvements of the invention provision is made to impart agreater resistance to the bending of the vertices of the side in whichis located the tongue for fastening with the single appendage of thetunnel body. This greater resistance is obtained by providing on theinner surface of each of the two vertices an embossed portion whichforms a protuberance on the opposed surface.

An attempt was made to omit the joining with the two equal appendagesand the bracket for retaining the latter, owing to the fact that thesaid joint is weak and a point at which the clips thus formed frequentlyfail.

This attempt consisted in arranging two transverse folds or steps on oneof the two ends of the strip, in order to create a deformation of thelevel of the upper surface, arranged in a circumferential direction,which forms a cavity of matching shape in the lower surface of that end,so that the facing ends of the two transverse appendages of the tunnelbody back against it.

By the said attempt, a weak connection was obtained, since on pullingstrongly the connected end of the metal strip coiled on itself to form aloop, this transverse double fold is undone by the strong stretchingpressure and thus smooths it, escaping from its engagement with the endsof the two transverse appendages of the tunnel body.

In order to avoid this, in the improvements two consecutive projectingstops are arranged, one on the lower surface of the corresponding lengthof the metal strip and the other on the opposed surface.

The two stops are obtained by embossing of the strip material of thesaid end.

The lower embossed portion acts as a stop in transverse action when itis pulled by the said end and butts against the edge of the singleappendage of the improvements.

The embossed portion projecting from the surface of the said end alsoacts as a stop for its elevation caused by the effect of strong tractionof the said end, which stop limits the degree of elevation by meetingthe inner surface of the superposed strip end at the said point.

This limitation of elevation, since the end rises up to a specificdegree of security, prevents the lower stop engaged with the transverseedge of the single appendage from escaping by smoothing of thetransverse folds which delimit the stepped seating of the transversesingle appendage.

For correct understanding of the subject of the invention, a descriptionwill now be given of a practical embodiment, by way of non-limitingexample, accompanied by a sheet of drawings in which:

FIG. 1 shows, uncoiled, the stamped metal sheet with which, once it isshaped, there is obtained the tunnel body for housing the tighteningscrew.

FIG. 2 shows a side view of the actual sheet.

FIG. 3 shows a side view from the opposite side of the actual sheet,partially in section.

FIG. 4 shows the body of the tunnel for housing the tightening screw, inlongitudinal section, so as to be able to observe the presence inside itof the said screw, the two superposed ends of the metal strip, thetransverse bent limb, and the single appendage in section simultaneouslyclamping both superposed ends.

FIG. 5 shows in a plan view from below the single appendage for clampingthe two superposed ends of the clip, shortened.

FIG. 6 shows from the outside the head of the tightening screw, thetunnel body, the bent limb and the two superposed, lengths of the endsof the metal strip.

FIG. 7 shows in plan the fastening of the transverse single appendage ofa lower edge of a side of the tunnel body, with the flange of the loweredge of the opposed side of the said tunnel body.

The improvements to the clips which are the subject of the invention areto those clips which are of the type formed by a metal strip (1), whichhas joined to one end (2) a tunnel body (3) for housing the screw (4)for tightening and loosening the constraining loop of the clip, whichtunnel body (3) has the ends of its descending vertical walls (5) and(6) separated sufficiently from the edges of the helical screw thread(7) of the tightening screw (4) introduced into the tunnel body (3), thetwo superposed lengths (1) and (2) of the metal strip, coiled on itselfto form a loop, clamped by a common one-piece appendage (8) arrangedfrom one side to the other of the open base of the tunnel body (3)covering the said space and its end projects beyond it, and is thendepressed against the opposed side of the lower part of the descendingwall of the tunnel body (3).

Moreover, before being bent back, there is inserted into its window afastening tongue which starts from the opposed edge of the tunnel bodyitself, and which drops down, and in the actual bending-back operationthat of the fastening tongue (9) is effected simultaneously.

In order to be able to impart greater resistance to the lateral thrustof the rear surface of the head of the tightening screw to the limbs (11and 12) bent in a transverse direction which form the fixing transversemember in the common type of this kind of clip, the inner vertices ofthe free ends of the said limbs are provided with a widening (13 and14).

This widening means that in the central part of the space in which theyare placed once they are bent, in relation to the body of the screw,they have a greater width. This greater width of course requires alarger seating space interposed between turns of the screw than existsat present in this kind of clip.

In order to be able to produce the said widening there is provided onthe body of the screw and near the upper end of the helical screw threada smooth annular space (16) of larger dimensions than the smooth helicalspace formed between turns which is left in the present industry toreceive such bent limbs.

The greater width means a greater degree of resistance to the pressurein a perpendicular direction provided by the rear surface (17) of thehead (18) of the screw (4) against the rear edge of the two bent limbs(11 and 12).

However, this enlargement of the edges of the free ends of the bentlimbs increases the possibility that at the said point each limb maybend under the perpendicular pressure of the second collar (17)permanently joined to the head (18) of the tightening screw (4).

In order to avoid irregularities in the lower surface of the head of thescrew this is smoothed.

In order to prevent the possibility of involuntary bending of the saidarea on each limb, a third projecting collar (19) is interposed betweenthe first collar (15) of the body (4) of the screw and the second collar(17) joined inseparably to the head (18) of the screw (4).

This intermediate collar (19) has a double slope, which forms a narrowrim.

The narrow rim is arranged against the wall of each transverse limb (11and 12) forming part of the fixing transverse member between the startof the limb and its end bearing against the first annular collar (15).

In this way the rim of the interposed collar (19), by its pressure in adirection from inside to outside, prevents the wall of the limb (11)from yielding and being able to bend inwards. Avoiding this bendingtowards the inside prevents the edge of each bent limb from beingseparated frown its support provided by the first collar (15).

Since the collar (15) is circular, and not helical as occurs at presentin that the rear edges (13 and 14) of the bent limbs (11 and 12) bearagainst an inclined surface, this support obtained by the circularcollar is completely perpendicular, not in an inclined position and theresult of the support is much more effective.

On the other hand, the greater width of the bent limbs (11 and 12) andespecially at their facing ends (13 and 14) allows the inclination ofone edge towards the other opposed edge to be more marked than thatexisting in the narrow bent limbs known in the industry.

Also with this arrangement there is obtained a larger surface ofinclination of the bent limb (11 and 12), which larger surface ofinclination also forms a larger contact surface between the outersurface of the limb and the inner surface of the superposed end (1) ofthe two ends joined at this point of coincidence and thus there isobtained a downward thrusting action of the projecting end (1),overcoming the negative tendency of the said free end to rise up.

With regard to the possibility of escape of the end (1) with which isengaged the fixing transverse member (8) of the base of the tunnel body(3) when the said end is subjected to a strong stretching tension, thereare arranged at the said end (2) two embossed portions (22 and 23)produced one behind the other, but each on one of its two opposedsurfaces, forming a protuberance (22) on the lower surface of the saidlength and immediately at the side, but on the upper surface of the samelength there is another embossed portion projecting upwards (23), thelower one (22) being a stop when, by stretching the end, the lattermeets and presses against the edge of the single appendage (8) fortransverse clamping of the two superposed lengths (1 and 2) of thestrip, while the upper stop (23) limits the rising movement of the saidfree end (2) of the metal strip in order to prevent the angle ofinclination of that end from being acute and, by its elevation, bringingabout the unclamping obtained by the face of the first lower stop (22)clamped with the lateral edge of the single appendage (8) and subsequentnegative smoothing of the two transverse steeps, parallel to each other,of the said end of the metal strip.

In addition to the two longitudinal slots near the edge of each side, inorder to form with them a reinforcing rib, two embossed portions (20 and21) are arranged against the inner surface of the end vertices of theside of the tunnel body forming two protuberances in the opposed faceseen, which prevent the negative elevation of said vertices by rubbingagainst hard objects, this independently of the two longitudinal slotsof the lower edge of each wall of the screw-carrying tunnel.

In addition to the bending back under pressure of the aforesaid singletransverse clamping appendage, in order to provide greater certainty ofnon-separation of the bent-back portion, there is arranged on theopposite edge from the start of the single appendage a perpendicularflange which, when the single appendage bends in a transverse direction,is inserted into a window provided in the surface of the singleappendage, and it bends and overlaps against the edges of the window,and then there is obtained the permanent simultaneous fastening underhigh pressure, in this way eliminating the bracket, its bending into ahook and the formation of the two rectangular flattened portions, oneach juxtaposed vertex of the two known appendages and also the weaknessoccurring in a transverse direction between the faces of the two equalappendages bent with respect to each other.

In order to prevent one of the ends of the two superposed lengths ofstrip from projecting laterally and thus the possibility of scratchesand cuts being caused during the manipulation of the clips, the vertexof one of the ends, the one which tends to project (1), is bevelled sothat its emergence disappears and in addition all the vertices areblunt.

It is understood that in the present case any details of constructionand finish which do not alter, change or modify the essence of theinvention may be varied.

What is claimed is:
 1. In a metal clamp comprising a metal strip havinga tunnel body attached at one end thereof with a space between thetunnel body and the strip, the tunnel body formed by a rectangular,stamped and embossed sheet, and at the other end of the strip aplurality of inclined, parallel, transverse slots located betweenopposed edges of the strip, the other end being constructed and arrangedfor insertion in the space between the tunnel body and the strip to forma loop of the strip coiled upon itself, the tunnel body receiving atightening screw having a helical screw thread and a head for moving thestrip through the space, superposing the strip upon itself,the tunnelbody including a base comprising two transverse appendages for conjointclamping of the superposed metal strip and a perpendicular bracket fortightening against juxtaposed, flattened vertices, the improvementcomprising forming the tunnel body from a sheet comprising a first endhaving opposed arms provided at ends thereof with a greater width by anenlargement of rear vertices of the ends for reinforcement of acorresponding rear vertex, the screw being adapted to fit saidenlargement by omission of the helical thread on a screw portion nearsaid head leaving a smooth annular surface thereon, said smooth annularsurface including adjacent the head a first annular collar concentricwith a further annular collar with a double slope which forms aconcentric narrow annular rim, a single one-piece appendage beingarranged at a base of the tunnel body for clamping the superposed metalstrip, at the opposite end of the metal strip there being provided twojuxtaposed embossed projecting stops arranged in sequence, one of saidstops projecting from a lower face of the strip and acting against thesingle transverse appendage, the other stop projecting from an upperface of the strip which limits rising movement when the other stop meetsthe superposed strip.
 2. The improvement of claim 1, wherein the firstcollar includes a smoothed lower surface to eliminate projectingirregularities.
 3. The improvement of claim 1, wherein the first collaris arranged on a final length of an upper end of the helical screwthread of the screw, the upper end being shortened forming a smoothannular area of the screw for receiving the opposed arms with enlargedends which thus bear laterally against the a uniform circular seatingand not against helical screw thread.
 4. The improvement of claim 1,additionally comprising a second concentric collar arranged between saidsmooth annular surface and the head of the tightening screw, the secondconcentric collar having a double sloped edge forming a narrow annularrim which presses against an inner surface of each of the opposed arms.5. The improvement of claim 1, wherein the sheet forming the tunnel bodyincludes a transverse rectangular extension on one side thereof having awindow centrally disposed therein, and a transversely extending tongueon the other side thereof, the tongue and window being constructed andarranged to interlock when the sheet is bent to form the tunnel.